1. Technical Field
The present disclosure relates to a wireless sensor device for detecting the motion of an object or a distance to the object by using a radio wave.
2. Related Art
A motion sensor or a distance detection sensor (hereinafter, referred to as a sensor device) is known for detecting the motion of an object or a distance to the object, which radiates a radio wave by supplying an antenna with a high frequency signal output from an oscillation circuit, receives a reflected wave from the object to convert the reflected wave into an electrical reception signal, and analyzes the reception signal (for example, refer to Japanese Unexamined Patent Application Publication No. 11-182109). The sensor device uses the principle (Doppler effect) that if a radio wave hits an object, the frequency of a reflected wave is slightly deviated from the frequency of the radiated wave, thereby measuring the motion or speed of the object from the size of the deviation (Doppler shift). Furthermore, since a radio wave is transmitted at the velocity of light through the air, the sensor device can measure the distance to the object from the delay amount (phase variation) of the reflected wave.
FIG. 6 is a diagram illustrating the principle (one example) of a Doppler radar. An RF generator 1 outputs a transmission signal including a continuous wave of a predetermined frequency, and a transceiver antenna 2 receives the transmission signal to radiate a radio wave. The transceiver antenna 2 receives a reflected wave from an object. A mixer 3 receives a part of the transmission signal and the reception signal to mix them. At this time, when the object is moving, the frequency of the reception signal is shifted from the frequency of the transmission signal by the Doppler phenomenon. The mixer 3 detects the difference between the frequency of the transmission signal and the frequency of the reception signal as an intermediate frequency (IF). That is, if the object is moving, since the frequency of the reflected wave changes, the mixer 3 outputs an IF signal corresponding to the Doppler shift. If the object is in a stationary state, since the frequency of the reflected wave does not change, the mixer 3 outputs only a DC signal. The IF signal and the DC signal are extracted from the output signal of the mixer 3 through a low-pass filter (LPF) 4, and are analyzed by a signal processing circuit 5, so that it is possible to determine the presence or absence of the motion of the object. Furthermore, a pulse Doppler radar or a digital modulation Doppler radar measures a delay time of a radio wave, which has reciprocated between an object and the pulse Doppler radar or the digital modulation Doppler radar, from a transmission time of a transmission signal and a reception time of a reflected wave, thereby detecting a distance to the object from the delay time.
The wireless sensor device mainly employs an object in a short distance (e.g., a distance smaller than several tens of m) as an object to be measured. However, the use of a part of a frequency band allocated to other wireless communication systems may be assumed. At the present time, a wireless communication system having a relatively narrow service area includes a wireless LAN, a WiMAX and the like.